1. Field of the Invention
The invention relates to an hydraulic mining device for forming a cavity in a borehole and, more particularly, to a device for forming a cavity in a coal seam to expose naturally occurring fractures, thereby enhancing the ability of methane gas trapped in the coal seam to be released through the fractures and recovered as an energy source.
2. Description of the Prior Art
U.S. Pat. No. 4,934,466 (hereinafter the '466 patent) describes a device for borehole hydraulic mining. The device generally includes first inner and second outer concentric pipelines for delivering fluid into a borehole and bringing a slurry (water, coal, sand and rock pieces) to the surface, respectively. A nozzle is secured to and hydraulically associated with the first pipeline, and extends through the second pipeline, for delivering the fluid into the borehole. An hydroelevator is secured to the lower end of the second pipeline, below the nozzle, for bringing the slurry to the surface.
Since the device extends all the way to the surface, assembly of the device would be very time consuming in a typical well because of the number of concentric strings of pipe necessary. In addition the device would require a very large drilling rig to handle the weight and associated pipe. Moreover, the device being so heavy frustrates insertion, operation and removal procedures.
This device is also relatively unitary with the functional portions such as nozzle and hydroelevator having set orientations. It is not possible to lengthen the device on site, or reverse the positions of the functional portions, e.g., nozzle and hydroelevator, if desired.
Further, since the entrance to the hydroelevator is fully open, there is no way to prevent relatively large coal pieces cut by hydraulic pressure from entering the hydroelevator. Blockage has been known to occur at this unrestricted entrance.
Also, since the entrance to the hydroelevator is open, i.e., a structural interruption is formed in the outer pipe, the device is supported at the hydroelevator only by the inner pipe. As the tool is rotated, e.g., if it becomes bound, torque could damage the inner pipe. This may result in a broken device incapable of being totally removed from the borehole, or if removable, may require extensive repairs. In either case, costs are increased.
In addition to the above drawbacks, the device of the '466 patent is not applicable specifically to enhancing recovery of methane from underground coal seams for the following reasons.
A critical problem in any well where methane is present is the chance the methane will escape to the surface uncontained and cause what is known in the industry as a blowout. Since the device of the '466 patent includes two steel concentric pipes extending all the way to the surface, too much steel is present at the surface to ensure cutting off the well by standard well blowout prevention equipment.
The prior art device also is not intended to receive standard oil field equipment which allows removal of the device from a well after the well starts producing methane. The device is instead intended to mine only minerals per se.
Thus, the prior art still does not provide the most efficient, safe and cost-effective hydraulic device for forming cavities in a coal seam so that methane recovery can be significantly enhanced.